Improvement of superconducting properties in high-critical-temperature oxide superconducting composite tapes using multiscale approach

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A new measurement method with higher accuracy was developed for mechanical properties of mesoscopic components in Bi2223/Ag/Ag alloy high-critical-temperature oxide superconductors. Then, mesoscopic stress and strain states under thermal and mechanical loading were analyzed for newly developed superconducting tapes with the controlled-overpressure sintering method. The correlation between the deformation and superconductivity were studied from the viewpoints of design optimization of composite tapes. The results are summarized as follows :(1)Direct measurement of elastic modulus of filament using Ag alloy removed tapeTensile tests were carried out for composite tapes from which Ag alloy component was completely removed. Bi2223 filaments carry almost the entire load in Ag alloy removed tapes because the yield strength and the coefficient of work hardening of Ag are quite low, and the volume fraction of Bi2223 filament is about twice as much as that of Ag. Then, the elastic modulus of Bi … More 2223 filament is simply calculated from the stress-strain curve of Ag alloy removed tapes where the Ag component stress is over the yield strength. In other words, this proposed method gives nearly direct measurement of Bi2223 filament properties. The obtained filament elastic moduli,160 to 170 GPa, are much higher than those of conventional Bi2223/Ag/Ag alloy tapes. The filament elastic modulus for conventional tapes also agreed with that obtained by extracted Bi2223 filaments. The controlled-overpressure process dramatically increases the filament modulus. The difference in filament moduli were minimal though the critical current increase.(2)Direct measurement of fracture strain of filament using Ag alloy removed tapeSince the yield strength and the coefficient of work hardening of Ag are quite low, the thermal residual strain of Bi2223 filament is estimated as 0.004% at most. This value is about 5% of the fracture strain of Ag alloy removed tapes. Then, the residual strain of filament can be ignored in the evaluation of the fracture strain of filament, and the fracture strain of filament is directly obtained by the fracture strain of Ag alloy removed tapes.(3)Accuracy of measured fracture strain of filament in compositesThe simultaneous measurement of mechanical and superconducting properties of tapes at 77K indicated that the filament fracture occurred slightly before the final tape fracture. Taking into account this fact and all of the thermal history, the stress-strain curves of composite tapes were summing up. Here, the stress-strain curves of Ag alloy were estimated by using Vickers hardness data of its component. The good agreement between experiment and simulation indicated that the filament fracture strain evaluated by the Ag alloy removed method agreed well with that in composite tapes.(4)Effect of processing method on fracture strain of filamentThe filament fracture strains, about 0.08%, are almost the same without respect to the fabrication method of Bi2223/Ag/Ag alloy tapes. Less